1. Field of the Invention
The present invention relates to a method and system for controlling the process of etching a structure on a substrate, for example, in semiconductor manufacturing. More particularly, it relates to a method for determining an endpoint of an etch process.
2. Description of Related Art
Plasma etch processes are commonly used in conjunction with photolithography in the process of manufacturing semiconductor devices, liquid crystal displays (LCDs), light-emitting diodes (LEDs), and some photovoltaics (PVs). Generally, a layer of radiation-sensitive material, such as photoresist, is first coated on a substrate and exposed to patterned light to impart a latent image thereto. Thereafter, the exposed radiation-sensitive material is developed to remove exposed radiation-sensitive material (or unexposed, if negative tone photoresist is used), leaving a pattern of radiation-sensitive material which exposes areas to be subsequently etched, and covers areas where no etching is desired. During the etch process, for example a plasma etch process, the substrate and radiation-sensitive material pattern are exposed to energetic ions in a plasma processing chamber, so as to effect removal of the material underlying the radiation-sensitive material in order to form etched features, such as vias, trenches, etc. Following etching of the features in the underlying material, the remainder of the radiation-sensitive material is removed from the substrate using an ashing or stripping process, to expose formed etched structures ready for further processing.
In many types of devices, such as semiconductor devices, the plasma etch process is performed in a first material layer overlying a second material layer, and it is important that the etch process be stopped accurately once the etch process has formed an opening or pattern in the first material layer, without continuing to etch the underlying second material layer.
For purposes of controlling the etch process various types of endpoint control are utilized, some of which rely on analyzing the chemistry of the gas in the plasma processing chamber in order to deduce whether the etch process has progressed, for example, to an underlying layer of a different chemical composition than the chemical composition of the layer being etched. Other processes may rely on direct in-situ measurements made of structures being etched. In the former group, optical emission spectroscopy (OES) is frequently used to monitor the chemistry of the gas in the plasma processing chamber. The chemical species of the gas in the plasma processing chamber are excited by the plasma excitation mechanism being used, and the excited chemical species produce distinct spectral signatures in the optical emission spectrum of the plasma. Changes in the optical emission spectrum due to, for example, clearing of a layer being etched, and exposing of an underlying layer on the substrate, can be monitored and used to precisely end, i.e. endpoint the etch process, so as to avoid etching of the underlying layer or formation of other yield defeating defects, such as undercuts, etc.
Depending on the types of structures being etched and the etch process parameters, the change of the optical emission spectrum of the plasma at the endpoint of the etch process may be very pronounced and easy to detect, or conversely subtle and very difficult to detect. For example, etching of structures with a very low open ratio can make endpoint detection difficult using current algorithms for processing optical emission spectroscopy (OES) data. Improvements are therefore needed to make etch endpoint detection based on optical emission spectroscopy (OES) data more robust in such challenging etch process conditions.